For example, all humans are members of the same species, Homo sapiens, but no two individuals are exactly alike. Even for twins, there are slight differences in their DNA. For the global population, there are many similarities and differences among people. For example, eye colour and blood type differ among individual Homo sapiens. Differences in these traits are due to genetic differences, or genetic variation. The human gene pool carries alternative alleles that affect blood type and many other traits. Other species also have variation in their gene pools. For example, apple trees are all members of one species, but the fruit produced by different trees can be red or yellow, hard or soft, sweet or tart, large or small. These differences are caused partly by genetic variation. When two or more alleles of a gene are present in a gene pool, the population is said to be polymorphic.

Population geneticists have studied the gene pools of many species of plants and animals. They have examined variation in obvious traits such as shape and colour. In many cases, they have also found genetic variation in the amino-acid sequences of proteins and the nucleotide sequences of DNA. For example, the fruit fly Drosophila melanogaster has an enzyme called alcohol dehydrogenase. There are two slightly different forms of alcohol dehydrogenase that can be distinguished by electrophoresis. The two forms differ by only one amino acid. The amino acid difference is caused by one nucleotide difference in the DNA. In natural populations all over the world, Drosophila melanogastergene pools are polymorphic for the alcohol dehydrogenase gene. The populations have both types of alleles and produce both varieties of alcohol dehydrogenase.